Use skip_checks instead of skipUnless in TestShelveInstance

All testcases in TestShelveInstance should be skipped if
CONF.compute_feature_enabled.shelve is false, so this is to
use skip_checks to avoid adding skipUnless decorator for
each testcase individually.

Change-Id: I41f57fc92cf0294e922b87667b84d4aa3ac87663
1 file changed